US4832939A - Barium titanate based dielectric compositions - Google Patents

Barium titanate based dielectric compositions Download PDF

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US4832939A
US4832939A US07/151,688 US15168888A US4832939A US 4832939 A US4832939 A US 4832939A US 15168888 A US15168888 A US 15168888A US 4832939 A US4832939 A US 4832939A
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Jameel Menashi
Robert C. Reid
Laurence P. Wagner
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    • C04B35/4684—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on oxide ceramics based on titanium oxides or titanates based on titanates based on alkaline earth metal titanates based on barium titanates based on BaTiO3 perovskite phase containing lead compounds
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Definitions

  • This invention relates to a method of producing barium titanate and barium titanate based dielectric compositions and, more particularly, relates to a method of hydrothermally synthesizing dispersible, submicron barium titanate and barium titanate compositions which have very narrow particle size distributions.
  • barium titanate makes it an especially desirable material from which capacitors, condensers, and other electronic components can be fabricated. Especially attractive is the fact that barium titanate's electrical properties can be controlled within a wide range by means of mixed crystal formation and doping.
  • the very simple cubic perovskite structure exhibited by barium titanate is the high temperature crystal form for many mixed oxides of the ABO 3 type.
  • This crystal structure consists of a regular array of corner-sharing oxygen octahedra with smaller titanium(IV) cations occupying the central octahedral B site and barium(II) cations filling the interstices between octahedra in the larger 12-coordinated A-sites.
  • This crystal structure is of particular significance since it is amenable to a plethora of multiple cation substitutions at both the A and B sites so that many more complex ferroelectric compounds can be easily produced.
  • Barium titanate's relatively simple lattice structure is characterized by the TiO 6 -octahedra which because of their high polarizability essentially determine the dielectric properties of the structure.
  • the high polarizability is due to the fact that the small Ti(IV) ions have relatively more space within the oxygen octahedra.
  • This cubic unit cell is stable only above the Curie point temperature of about 130° C. Below 130° C., the Ti(IV) ions occupy off-center positions. This transition to the off-center position results in a change in crystal structure from cubic to tetragonal between temperatures of 5° C. and 130° C., to orthorhombic between -90° C. and 5° C. and finally to rhombohedral at temperatures less than -90° C. Needless to say, the dielectric constant and strength also decreases relative to these temperature and crystal structure changes.
  • the dielectric constant of barium titanate ceramic has a strong temperature dependence and exhibits a pronounced, maximum dielectric constant at or around the Curie point
  • pure BaTiO 3 is rarely used in the production of commercial dielectric compositions.
  • additives are employed to upgrade the dielectric properties of barium titanate.
  • the Curie temperature can be shifted to lower temeratures and broadened by effecting a partial substitution of strontium and/or calcium for barium and of zirconium and/or tin for titanium, thereby resulting in materials with a maximum dielectric constant of 10,000 to 15,000 at room temperature.
  • the Curie temperature can be increased by a partial substitution of lead(II) for barium.
  • barium titanate based dielectric powders are produced either by blending the required pure titanates, zirconates, stannates and dopants or by directly producing the desired dielectric powder by a high temperature solid state reaction of an intimate mixture of the appropriate stoichiometric amounts of the oxides or oxide precursors (e.g., carbonates, hydroxides or nitrates) of barium, calcium, titanium, etc.
  • the pure titanates, zirconates, stannates, etc. are also, typically, produced by a high temperature solid phase reaction process. In such calcination processes the required reactants are wet milled to accomplish the formation of an intimate mixture.
  • the resulting slurry is dried and calcined at elevated temperatures, ranging from about 700° to 1200° C., to attain the desired solid state reactions. Thereafter, the calcine is remilled to produce a dispersible powder for use in making green bodies.
  • the processes for producing barium titanate by solid phase reactions are relatively simple; nevertheless, they do suffer from several disadvantages.
  • the milling steps serve as a source of contaminants which can adversely affect electrical properties. Compositional inhomogenieties on a microscale can lead to the formation of undesirable phases such as barium orthotitanate, Ba 2 TiO 4 , which can give rise to moisture sensitive properties.
  • undesirable phases such as barium orthotitanate, Ba 2 TiO 4 , which can give rise to moisture sensitive properties.
  • substantial particle growth and interparticle sintering occur.
  • the milled product consists of irregularly shaped fractured aggregates which have a wide size distribution ranging from about 0.2 up to 10 microns.
  • barium titanate has been developed from molten salts, by hydrolysis of barium and titanium alkoxides dissolved in alcohol and by the reaction of barium hydroxide with titania both hydrothermally and in aqueous media.
  • barium titanate based compositions or coforms Since the barium titanate products produced by some of these processes have been shown to have morphologies approaching those desired here attempts have been made to extend these same methods to the production of barium titanate based compositions or coforms.
  • B. J. Mulder discloses in an article entitled "Preparation of BaTiO3 and Other Ceramic Powders by Coprecipitation of Citrates in an Alcohol", Ceramic Bulletin, 49, No. 11, 1970, pages 990-993, that BaTiO 3 based compositions or coforms can be prepared by a coprecipitation process.
  • aqueous solutions of Ti(IV), Zr(IV) and/or Sn(IV) citrates and formates of Ba(II), Mg(II), Ca(II), Sr(II) and/or Pb(II) are sprayed into alcohol to effect coprecipitation.
  • the precipitates are decomposed by calcination in a stream of air diluted with N 2 at 700 °-800° C. to give globular and rod shaped particles having an average size of 3 to 10 microns.
  • Barium titanate based coforms have been prepared by precipitation and subsequent calcination of mixed divalent alkaline earth metal and/or Pb(II) titanyl and/or zirconyl oxalates as disclosed by Gallagher et al. in an article entitled "Preparation of Semi-Conducting Titanates by Chemical Methods", J. Amer. Ceramic Soc., 46, No. 8, 1963 pages 359-365. These workers demonstrated that BaTiO 3 based compositions in which Ba is replaced by Sr or Pb in the range of 0 to 50 mole percent or in which Ti(IV) is replaced by Zr(IV) in the range of 0 to 20 mole percent may be produced.
  • BaTiO 3 based coforms can be synthesized by heating a solution of a titanium chelate or a titanium alkoxide, an alkaline earth salt and a lanthanide salt to form a semisolid mass. The mass is then calcined to produce the desired titanate coform.
  • the reactants dissolve in the molten solvent and precipitate as an alkaline earth titanate, zirconate or a solid solution having the general formula Ba x Sr.sub.(1-x) Ti y Zr.sub.(1-y) O 3 .
  • the products are characterized as chemically homogeneous, relatively monodisperse, submicron crystallites.
  • Matsushita et al. in European patent publication No. 014551 demonstrated that dilute slurries of hydrous titania can be reacted with Ba(OH) 2 and/or Sr(OH) 2 by heating to temperatures up to 110° C. to produce either BaTiO 3 or Sr-containing coforms.
  • the morphological characteristics of these coforms appear to be comparable with those of this invention. The method, however, is again limited to producing only Sr-containing coforms.
  • a publication of the Sakai Chemical Industry Company entitled "Easily Sinterable BaTiO 3 Powder", by Abe et al. discloses a hydrothermal process for synthesizing a barium titanate based coform with the formula BaTi.sub.(1-x) Sn x O 3 .
  • a 0.6M Ti.sub.(1-x) Sn x O 2 slurry prepared by neutralizing an aqueous solution of SnOCl 2 and TiCl 4 , is mixed with 0.9M Ba(OH) 2 and subjected to a hydrothermal treatment at 200° C. for at least five hours.
  • Abe et al. imply the slurry was heated to temperature.
  • Acetic acid is added to the resultant slurry in order to adjust the pH to seven and a washed product having a Ba/Ti mole ratio of 0.99 was recovered.
  • barium oxalate was added to the product.
  • the BaTiO 3 product produced by the same process had a surface area of 11 m 2 /g, a particle size of 0.1 micron and appeared to be dispersible.
  • the Sn-containing coforms have comparable morphologies and are thus comparable with those of this invention.
  • Abe et al. is limited in that it teaches only that Sn(IV) can be synthesized into a barium titanate coform.
  • the present invention is a method of hydrothermally synthesizing stoichiometric, submicron, dispersible doped and undoped barium titanate and dielectric compositions of barium titanate which have very narrow particle size distributions.
  • barium titanate powder is produced by introducing a solution of 0.5 to 1.0 molar Ba(OH) 2 heated to a temperature between 70°-110° C., preferably 70°-90° C., into a vigorously stirred slurry of a high surface area hydrous titania at a temperature ranging between 60° C. and 150° C. at a constant rate over a time period of less than five minutes.
  • the Ba(OH) 2 introduction process continues until the Ba/Ti mole ratio in the slurry is between 1.1 to 1.3.
  • the slurry is then held at temperature for 10 to 30 minutes so that 95 to 98 percent of the TiO 2 is converted to BaTiO 3 .
  • the slurry is then heated to an elevated temperature, preferably at least 175° C., to ensure complete conversion of the tetravalent hydrous oxide to a stoichiometric perovskite.
  • the slurry is pressure filtered to give a cake of stoichiometric BaTiO 3 containing 80 to 85 weight percent solids.
  • the product is then washed with either water or a 0.01 to 0.02M Ba(OH) 2 solution.
  • the wet cake is then dried resulting in a high purity, stoichiometric barium titanate powder having a primary particle size in the range between 0.05 and 0.4 micron with a very narrow particle size distribution.
  • various submicron, dispersible barium titanate based coforms are produced hydrothermally in which the divalent barium of the barium titanate is partially replaced by one or more divalent cations and/or the tetravalent titanium is partially replaced by one or more tetravalent cations.
  • the coforms do not contain either Pb(II) or Ca(II)
  • a heated solution of Ba(OH) 2 containing the requisite amount of Sr(OH) 2 is added to a vigorously stirred slurry of the hydrous oxides of TiO 2 and/or SnO 2 , ZrO 2 and HfO 2 in a fixed time interval at a range of temperatures between 60° and 200° C.
  • the slurry is then heated to an elevated temperature so that any remaining unreacted hydrous oxides combine with the soluble divalent cation hydroxides.
  • the oxide or hydroxide of Pb and/or Ca(OH) 2 are first hydrothermally treated with a stoichiometric excess of the tetravalent cations at temperatures up to 200° C.
  • PbO or Pb(OH) 2 or Ca(OH) 2 which, unlike Sr(OH) 2 or Ba(OH) 2 , are relatively insoluble in aqueous media at temperatures up to 200° C., combine with the tetravalent hydrous oxides to form perovskites.
  • the molar ratio of the relatively insoluble divalent cation oxides or hydroxides to tetravalent hydrous oxides is less than 0.4 and, preferably less than 0.3, after perovskite formation substantial amounts of unreacted hydrous oxides remain.
  • the slurry is adjusted to a temperature between 60° C. and 150° C. and then a heated solution containing the requisite concentrations of Ba(OH) 2 and Sr(OH) 2 is introduced at a constant rate over a specified time period.
  • the resulting slurry is held at temperature for 10 to 30 minutes and then, if necessary, heated to an elevated temperature to ensure complete conversion of the tetravalent hydrous oxides to stoichiometric perovskites. Because the CaTiO 3 and PbTiO 3 perovskites can undergo displacement in the presence of excess Ba(OH) 2 , the stoichiometric excess of Ba(OH) 2 employed and the final temperature to which the slurry is heated are carefully controlled.
  • both barium titanate and the barium titanate based coforms are uniformly doped in the hydrothermal synthesis process with small amounts of one or more of a variety of dopants.
  • Typical dopants are those described in the literature and include niobium(V), lanthanum(III), yttrium(III), nickel(II), manganese(II), iron(III) and cobalt(II).
  • the doped products are produced by addition of the requisite amounts of the dopant or dopants, either as a high surface area hydrous oxide wet cake or as a solution of their soluble salts, to the tetravalent hydrous oxide slurry prior to initiation of the synthesis process.
  • the soluble dopant salts employed are those having anions, such as nitrates, formates and acetates, which can be eliminated during a subsequent sintering operation either by decomposition or by oxidation. Thereafter, the slurry is treated in an analogous manner to those employed in the synthesis of either barium titanate or one of the various coforms.
  • the FIGURE is a transmission electron micrograph of the product produced by example 5.
  • the present invention is a method of producing stoichiometric, unaggregated, dispersible, submicron doped and undoped barium titanate powder and doped and undoped coforms of barium titanate powder which have a very narrow particle size distribution.
  • the preferred barium titanate powder and coforms of barium titanate powder have the general formula:
  • the powder is uniquely characterized by its high purity, fine submicron size, lack of aggregation and very narrow particle size distribution.
  • the resulting product is a BaTiO 3 coform where y mole fractions of Ti(IV) in BaTiO 3 have been replaced by Sn(IV) to give a product with the nominal formula BaTi.sub.(1-y) Sn y O 3 .
  • the coform has the composition Ba.sub.(1-x) Pb x TiO 3 .
  • high purity, unaggregated, dispersible, submicron barium titanate powder is produced by introducing, over a fixed period of time, a hot solution of 0.2 to 1.0 molar Ba(OH) 2 into a vigorously stirred slurry of a high surface area hydrous titania having a temperature ranging from 50° to 200° C. and, more preferably, between 60° to 150° C.
  • a hot solution of 0.2 to 1.0 molar Ba(OH) 2 into a vigorously stirred slurry of a high surface area hydrous titania having a temperature ranging from 50° to 200° C. and, more preferably, between 60° to 150° C.
  • BaTiO 3 precipitation is initiated by a nucleation burst.
  • the conditions are such that nuclei growth rather than further nucleation occurs. Since little additional nucleation occurs, a product with a narrow primary particle size distribution results.
  • the Ba(OH) 2 addition process is continued until the Ba/Ti mole ratio in the slurry is greater than unity and is typically between 1.1 to 1.3.
  • the slurry is then held at temperature for 10 to about 30 minutes, with lower processing temperatures requiring the longer hold times, whereby 95 to 98% of the TiO 2 is converted to BaTiO 3 .
  • the slurry may be heated to 150° to 200° C. and preferably to at least 175° C.
  • This final slurry typically contains about 0.2 moles of BaTiO 3 per liter of solution.
  • the solution also contains 0.02 to 0.06 M/L Ba(OH) 2 .
  • the slurry is pressure filtered to give a cake of stoichiometric BaTiO 3 containing over 75 weight percent, and typically between 80 to 85 weight percent, solids. Since washing, depending on wash water volume, wash water pH and temperature, can reduce the Ba/Ti mole ratio in the product, it is preferred to wash the product with a dilute Ba(OH) 2 solution, say a 0.01 to 0.02M Ba(OH) 2 solution. In view of the high solids content of the cake, the quantity of Ba in the volume of wash solution trapped in the cake will contribute less than 0.2 mole percent Ba to the Ba-content of the BaTiO 3 product. Alternatively, if the residual Ba(OH) 2 content of the slurry is small, the washing step can be entirely eliminated.
  • the particle size of the barium titanate powder produced according to the method of the present invention may be controlled.
  • the formation of a large number of nuclei indicates that a fine particle size will be attained; conversely, the formation of fewer nuclei will result in a barium titanate powder having a larger primary particle size.
  • the time interval for the addition of the Ba(OH) 2 should be relatively short, preferably less than 0.3 minutes.
  • the primary particle size of the barium titanate powder correspondingly increases. At prolonged addition times, however, such as 12 minutes at 120° C., for example, aggregation of the primary particles can occur which adversely affects product dispersibility.
  • the reaction temperature should also be controlled to ensure that an optimal number of BaTiO 3 nuclei are formed. A reduced reaction temperature favors the formation of fewer nuclei and thus the formation of products with larger primary particle sizes. A reaction temperature ranging from 50° to 200° C.
  • the preferred method for producing the multi-component coforms of barium titanate is similiar to that utilized to synthesize simple barium titanate.
  • a heated solution of Ba(OH) 2 or a heated solution of Ba(OH) 2 containing the requisite amount of Sr(OH) 2 is added in a fixed time interval to a vigorously stirred slurry of the hydrous oxides of tetravalent cations at a prescribed temperature in the range of 60° to 200° C.
  • the number of nuclei formed increases with increase in rate of hydroxide addition, temperature, hydrous oxide surface area and reactant concentrations.
  • the soluble divalent cation hydroxide may contain some quantities of unreacted hydrous oxides.
  • the unreacted hydrous oxides combine with the soluble divalent cation hydroxides mostly on the surfaces of the particles already present.
  • coforms with primary particle sizes in the range of 0.05 to 0.4 micron with narrow size distributions are produced.
  • the oxide or hydroxide of Pb and/or Ca(OH) 2 are first hydrothermally treated with a stoichiometric excess of the hydrous oxides of the tetravalent cations at temperatures up to 200° C..
  • Both nitrogen surface area (BET) measurements and transmission electron micrographs of the products formed indicate that they consist of extremely fine sized (sizes below about 0.02 micron) Pb(II) and/or Ca(II) containing perovskites in combination with a mass of high surface area unreacted hydrous oxides.
  • the slurry is typically cooled to a prescribed temperature and addition of soluble divalent cation hydroxide is initiated. Shortly after hydroxide addition is commenced, it is believed that a nucleation burst occurs both in the medium but primarily on the surfaces of the Pb(II) and/or Ca(II) perovskites already present. As additional hydroxide is introduced particle growth, rather than additional nucleation, occurs. Again, coform particle size is dependent on the same set of parameters as those controlling the size of BaTiO 3 .
  • a vigorously stirred slurry of the relatively insoluble divalent oxides or hydroxides, that is of Pb(II) and Ca(II), and the total amount of the hydrous oxides of TiO 2 , SnO 2 , ZrO 2 and HfO 2 , preferably coprecipitated in the appropriate molar ratio, is hydrothermally treated at temperatures up to 200° C.. At the elevated temperature the relatively insoluble divalent metal oxides and/or hydroxides combine with the tetravalent hydrous oxides to form one or more perovskites.
  • the molar ratio of the relatively insoluble divalent cations to tetravalent hydrous oxides is less than 0.4 and, preferably less than 0.3, after perovskite formation substantial amounts of unreacted hydrous oxides remain.
  • the temperature of the slurry, containing the mixture of perovskites and unreacted hydrous oxides is adjusted to a prescribed temperature between 50° and 200° C. but more preferably between 60° and 150° C. and then a heated solution containing the requisite amounts of Ba(OH) 2 and Sr(OH) 2 is introduced at a constant rate over a specified time period.
  • reaction (1) The TiO 2 formed rapidly reacts with the excess Ba(OH) 2 , as shown by reaction (1), to form BaTiO 3 .
  • reaction (1) and (4) do not lead to the formation of a non-stoichiometric product, they indicate that the Pb-content of the coform can only be controlled when the OH- - concentration in solution is controlled.
  • the stoichiometric excess of Ba(OH) 2 should be relatively small and, within limits, well controlled.
  • the final treatment temperature employed will represent a compromise between coform composition, treatment temperature, time at temperature and stoichiometric excess of Ba(OH) 2 .
  • the requisite amounts of the dopant or dopants are intimately mixed with the tretavalent hydrous oxide or oxides.
  • Intimate mixing can be accomplished by one of a variety of methods.
  • the dopants can be coprecipitiated with the tetravalent hydrous oxides. This method, however, is not applicable to all dopants because some, like Co(II) and Ni(II), will be incompletely precipitated during the ammoniacal neutralization process as a result of the formation of complex amines.
  • the dopants can be precipitated as high surface hydrous oxides which can then be slurred with the tetravalent hydrous oxides.
  • the dopants can be added as solutions of acetates, formates or nitrates to the tetravalent hydrous oxides.
  • the dopant or dopants typically, represent less than five, and more preferrably less than three, mole percent of the tetravalent hydrous oxides.
  • the slurry depending on product composition, is treated in analgous manner to those described above for barium titanate or one of its various coforms. After filtration and washing high solids cakes are obtained.
  • the dopant or dopants represent less than 5 mole percent of the barium titanate or the coform, their product morphology is similar to those formed in the absence of dopants.
  • the same combinations of treatment parameters used to alter the median primary particle sizes of the undoped products can be used to alter the median primary particle sizes of the doped products.
  • Product stoichiometry will depend on the dopant or dopants employed. Some dopants, such as Mn(II) or Co(II) do not react under the hydrothermal synthesis conditions described here with either the divalent alkine earth and lead(II) cations or the tetravalent hydrous oxides. Accordingly, in these cases a stoichiometric barium titanate or a stoichiometric coform containing the dopant, as an oxide or as a hydrous oxide, is formed.
  • dopants such as Mn(II) or Co(II) do not react under the hydrothermal synthesis conditions described here with either the divalent alkine earth and lead(II) cations or the tetravalent hydrous oxides. Accordingly, in these cases a stoichiometric barium titanate or a stoichiometric coform containing the dopant, as an oxide or as a hydrous oxide, is formed.
  • dopants under the hydrothermal synthesis conditions employed here, may react with either the tetravalent hydrous oxides or the alkaline earth and Pb(II) cations.
  • Nb(V) reacts with Ba(II) to form BaNb 2 O 6 .
  • product stoichiometry can be varied by a variety of methods know to those skilled in the art.
  • the barium content of the product can be increased by addition of the requisite amount of a solution of ammonium carbonate or, where applicable, by controlling the extent of the displacement reactions.
  • Image analysis was used to determine product primary particle size and primary particle size distribution of the powders produced.
  • 500 to 1000 particles were sized in a plurality of TEM fields so as to obtain the equivalent spherical diameters of the primary particles.
  • Two or more touching particles were visually disaggregated and the sizes of the individual primary particles were measured.
  • the equivalent spherical diameters were used to compute the cumulative mass percent distribution as a function of primary particle size.
  • the median particle size, by weight, was taken to be the primary particle size of the sample.
  • the quartile ratio, QR defined as the upper quartile diameter (by weight) divided by the lower quartile diameter, was taken as the measure of the width of the distribution.
  • Monodisperse products have a QR value of 1. Products with QR values ranging from 1.0 to about 1.5 are classified as having narrow size distributions; those with QR values ranging from 1.5 to about 2.0 have moderately narrow distributions while those with values substantially greater than 2.0 have broad size distributions.
  • the equivalent spherical diameters were also used to compute surface areas from TEM data. Comparable TEM and N 2 surface areas indicate the primary particles are essentially nonporous.
  • the powders were dispersed by a 15 to 30 minutes sonification in either water containing 0.08 g/L sodium tripolyphosphate at pH 10 or in isopropanal containing 0.08 or 0.12 weight percent Emphos PS-21A (Witco Organics Division, 520 Madison Ave., New York).
  • Particle size determined by image analysis and by sedimentation depend on different principles. For this reason an exact correspondence in size by these two methods is not always obtained.
  • touching particles are visually disaggregated.
  • bound or flocculated particles act as single entities. These entities arise because of the existence of some bonding (e.g., necking) between the primary particles to give cemented aggregate which cannot be readily broken down during the sonification process and because of less than optimum dispersion stability which leads to some flocculation.
  • QR values determined by sedimentation are expected, and found, to be larger than those found by image analysis. It is likely that under optimum dispersion conditions the QR value will be within the two sets of values cited here.
  • the mass fraction of the product having a Stokes diameter greater than one micron was used as a measure of the amount of hard-to-disperse aggregates.
  • a product was classified as being dispersible if the bulk of the primary particles in the TEM's were present as single particles. When substantial necking was observed the product was classified as aggregated.
  • Produce composition and stoichiometry were determined by elemental analysis using inductively coupled plasma spectroscopy after sample dissolution. The precision of the analyses was about ⁇ 1%.
  • the molar ratio of the sum of the divalent cation to the sum of tetravalent cations, X(II)/Y(IV), was used as a measure of stoichiometry. Products were taken to be stoichiometric when X(II)/Y(II) 1.000 ⁇ 0.015.
  • Reagent grade chemicals or their equivalents were used throughout. The purity of the final powder is dependent, in part, on the purity of the reactants employed.
  • the reagent grade Ba(OH) 2 .sup. ⁇ 8H 2 O employed contained about 0.2 weight percent Sr. Since, as will be shown, Sr in the reactant tends to concentrate in the product, a knowledge of the level of Sr present in Ba(OH) 2 .sup. ⁇ 8H 2 O is important.
  • Ba(OH) 2 and/or Sr(OH) 2 solutions maintained at 70°-100° C., were filtered prior to use to remove any carbonates present.
  • CaCO 3 was calcined at 800° C. to give CaO. The latter compound when contacted with water gives Ca(OH) 2 .
  • Pb(OH) 2 was prepared by neutralizing a Pb(NO 3 ) 2 solution with NH 3 . The washed hydroxide wet cake was used in subsequent experiments.
  • Hydrous oxides of TiO 2 , SnO 2 and ZrO 2 were prepared by neutralizing aqueous solutions of their respective chlorides with NH 3 at ambient temperatures. The products were filtered off and washed until chloride-free (as determened by AgNO 3 ) filtrates were obtained. The surface areas of the hydrous oxides, determined after drying at 110° C., were about 380, 290 and 150 m 2 /g for TiO 2 , SnO 2 and ZrO 2 , respectively.
  • coprecipitates of hydrous TiO 2 and ZrO 2 or hydrous TiO 2 and SnO 2 were prepared by neutralizing aqueous solutions of the chlorides of Ti(IV) and Sn(IV) or Ti(IV) and Zr(IV).
  • the filtered Ba(OH) 2 or Ba(OH) 2 and Sr(OH) 2 solutions were introduced into the autoclave either by means of a high pressure pump or by rapidly discharging a solution of the hydroxide or hydroxides, contained in a heated bomb, into the autoclave by means of high pressure nitrogen.
  • the contents of the autoclave were stirred by means of a one inch diameter turbine type stirrer operated at 1500 RPM throughout the synthesis process.
  • the slurries were, typically, transferred to a pressure filter without exposure to air, filtered and then dried either under vacuum or under nitrogen at 100° to 110° C.
  • the hydrous TiO 2 employed had an initial surface area of about 380 m 2 /g.
  • hydrothermal treatment of the hydrous TiO 2 to the required specified temperature decreases hydrous TiO 2 surface area. The magnitude of this decrease increases with increase in temperature, time at temperature and slurry pH.
  • hydrous TiO 2 surface area declined to about 300 m 2 /g at 150° C. and to about 150 m 2 /g at 200° C.
  • the hydrous TiO 2 surface area was reduced to 54 m 2 /g by a preliminary hydrothermal treatment of the hydrous TiO 2 at 200° C. in the presence of NH 4 OH for several hours.
  • a TEM of a sample of Example V demonstrated that the primary particles are substantially spherical in shape and uniform in size. Although the majority of the primary particles were unaggregated, a few firmly bonded doublets, triplets, etc. were also present. This TEM, after adjustment for magnification, was typical of those obtained for all products having QR values, by image analysis of about 1.3. The TEM of Example I, conversely, showed the presence of extensive necking between the primary particles.
  • Example IX The data in the table indicate that the solid product of Example IX is stoichiometric but that the solid product of Example X has a slightly high divalent cation content.
  • the divalent cation excess is attributed to a somewhat larger than desired Ba(OH) 2 concentration in the filtrate increasing the divalent cation content of the product by mother liquor entrapment and, very probably, by Ba(OH) 2 adsorption. Washing the solids with either 0.01 to 0.02M Ba(OH) 2 or with CO 2 -free ammoniated water would reduce the divalent cation content of the product so that a stoichiometric product would result.
  • the Sr/Ba mole ratios in the feeds are considerably larger than those in the filtrates. Also, the Sr contents of the filtrates are very small. This means that Sr concentrates in the solid phase and, for this reason, the Sr/Ba mole ratios in the products, 0.058 in Example IX and 0.245 in Example X, are larger than the comparable values in the feeds.
  • Example X X-ray diffraction for the solid of Example X indicated that a solid solution of BaTiO 3 and SrTiO 3 was present. Comparison of N 2 surface areas of the products of Examples V, IX and X indicates that as the Sr content of the hydroxide is increased product primary particle size decreases. Products with comparable primary particle sizes can be readily obtained by varying the same combination of treatment variables as was used to vary the primary particle size of BaTiO 3 . Finally, electron micrographs of these Sr-containing coforms demonstrated that their morphologies, other than for the small differences in particle size, were comparable to that of BaTiO 3 shown in FIG. 1.
  • a series of Zr-containing coforms were synthesized by treating 0.64 L of preheated slurries of hydrous TiO 2 and ZrO 2 , prepared either by mixing the individual hydrous oxides or by coprecipitation, with 0.46 L of preheated 0.52 to 0.6M Ba(OH) 2 .
  • the synthesis procedure was identical with those described above.
  • the quantities of hydrous oxides, the synthesis temperatures and Ba(OH) 2 addition times employed as well as the Ba(II) contents of the filtrates obtained are summarized in Table IV.
  • Example XVI only hydrous ZrO 2 was used and pure BaZrO 3 was synthesized. The characteristics of the solid products are shown in Table V.
  • the Zr(IV)/Ti(IV) mole ratio in the products range from 0.117 to 0.235.
  • the values given in Table IV are based on cake weights and their contained solids. Since only trivial amounts, at the ppm level, of Zr(IV) or Ti(IV) were detected in the filtrates, the Zr(IV)/Ti(IV) mole ratios in the two tables should be identical.
  • Example XVI BaZrO 3 was synthesized under conditions which result in the formation of about 0.06 micron BaTiO 3 or BaTiO 3 based coforms. Such products, typically, have N 2 surface areas of about 16 m 2 /g. With BaZrO 3 , however, product primary particle size was found to be about 1.5 micron, product surface area was 2.8 m 2 /g and TEM data indicated that the product was aggregated.
  • the Sn-containing coforms as shown by the X(II)/Y(IV) mole ratios in Table VII, contain a small excess of the divalent cation. Since the filtrates have rather high Ba-contents, Table VI, and since the products were filtered but not washed, the excess divalent cation contents are attributed to adsorbed Ba(OH) 2 . Experimental data, for other coforms, indicate that the divalent cation excess can be readily reduced to give stoichiometric products by washing. The data of Abe et al. are in support of this contention.
  • Example XX X-ray diffraction analysis indicated that the product of Example XX consists of barium hexahydrostannate, BaSn(OH) 6 .
  • the only crystalline phase found was that of BaTiO 3 .
  • STEM scanning transmission electron microscope
  • a CaTiO 3 product containing an excess of Ca(OH) 2 was synthesized. This was accomplished by hydrothermal treatment of a 1.0 L slurry containing 0.5 moles hydrous TiO 2 and 0.55 moles Ca(OH) 2 . Analytical data showed that the product had a surface area of 14.4 m 2 /g and a Ca(II)/Ti(IV) mole ratio of 1.04. About 0.2 moles of this product (27.2 g on a dry basis) was dispersed in 0.6 L of water and heated to 80° C. Thereafter 0.4 L of 0.37M Ba(OH) 2 was added. The resulting slurry was held at 80° C. for 90 minutes and sampled. The remaining slurry was next heated to 120° C., held at temperature for 60 minutes and sampled. This procedure was repeated at 150° C. and at 200° C. The slurry samples were filtered and the filtrates and dried solids were analyzed.
  • the data in the table demonstrate that displacement reaction (2) occurs at 200° C. until quite low Ba-concentrations are attained.
  • the Ca(OH) 2 formed is fairly insoluble and remains in the solid phase so that, as shown by the X(II)/Y(IV) mole ratio, a non-stoichiometric product is formed.
  • the displacement reaction is diffusion controlled and only at temperatures above about 150° C. is the diffusion coefficient of Ba(II) in CaTiO 3 sufficiently large that the extent of reaction (2) becomes large. It is expected that as the Ba(II) concentration in the aqueous phase declines the rate of reaction is reduced and becomes negligible at sufficiently low Ba(OH) 2 concentrations.
  • Example XXI The feasibility of preparing a Ca-containing coform under conditions where, after BaTiO 3 formation, the Ba(OH) 2 concentration in the aqueous phase is small was investigated.
  • the Ca-containing coform was prepared by hydrothermal treatment of 0.67 L of a slurry containing 0.21 moles of hydrous TiO 2 and 0.042 moles of Ca(OH) 2 to 200° C. The slurry was then cooled to 120° C. and 5% of the slurry (0.032 L) was withdrawn for characterization. Thereafter, 0.46 L of 0.41M Ba(OH) 2 was added in 3.1 minutes. The resulting slurry was held at 120° C. and sampled at 2, 10, 20 and 60 minutes after Ba(OH) 2 addition.
  • the slurry temperature was then raised to 150° C., held for 60 minutes and sampled. Next, it was raised to 200° C., sampled, held at 200° C. for 30 minutes and resampled. All samples were filtered and the divalent cation concentrations in the filtrates were determined. The filter cakes were dried and their surface areas and nominal stoichiometries determined. The results obtained are summarized in Table IX.
  • the solid phase of the initial sample, taken prior to Ba(OH) 2 addition, has a N 2 surface area of 291 m 2 /g. Since treatment of pure hydrous TiO 2 to 200° C. results in fairly substantial decreases in surface area, the present surface area measurement confirms that the crystallite size of the calcium titanate, formed in the presence of hydrous TiO 2 , is very small (less than about 0.02 micron). A TEM of the sample is in support of this contention.
  • the Ba-content of the solid phase increases and that of the filtrate decreases with time at 120° C.
  • the initial Ba concentration assuming no reaction, is estimated to be about 25.5 g/L.
  • Two minutes after Ba(OH) 2 addition the Ba-content of the liquid phase decreases to 15.9 g/L and about 40% of the TiO 2 is converted to BaTiO 3 . Accordingly, it is apparent that when the Ba(OH) 2 concentration in the aqueous phase is large, the rate of BaTiO 3 formation at 120° C. is rapid. As the Ba(OH) 2 concentration declines and the TiO 2 is transformed to BaTiO 3 the reaction rate decreases.
  • Example XXI A TEM of the solid phase of Example XXI, taken after 1 minute at 200° C., indicated that the product had a primary particle size of 0.15 micron, a narrow size distribution and that the product was dispersible. Product primary particle size determined from surface area, 0.13 micron, is in good agreement with the TEM primary particle size.
  • the slurry filtrate composition and the solid phase surface areas and nominal stoichiometries are listed in Table X.
  • Essentially stoichiometric products were formed.
  • TEM data showed that the primary particle size of the product of Example XXII was about 0.12 micron and that Example XXIII was 0.06 micron.
  • the TEM's, after adjusting for magnification were essentially identical to the TEM's of the pure barium titanate produced in Examples II-VIII.
  • X-ray diffraction showed that the only crystalline phase present in these Ca-containing coforms was BaTiO 3 .
  • a STEM - energy dispersive X-ray analysis of the product of Example XXII however, showed that each primary particle contianed Ca, Ti and Ba at fairly comparable levels.
  • PbTiO 3 products were prepared by hydrothermal treatment of a slurry containing an equi-molar mixture of PbO and hydrous TiO 2 , each present at a concentration of 0.33 moles/L, to 200°°C.
  • the slurry was sampled as soon as a temperature of 200° C. was attained and after aging for two hours at 200° C.
  • the N 2 surface area of the solid phase in the slurry samples decreased from 20.4 to 11.8 m 2 /g on aging, presumably as a result of Ostwald ripening.
  • TEM data showed that the products consisted of reasonably uniform but aggregated thin rectangular platelets.
  • a TEM of the product of Example XXIV did not show the presence of particles having the characteristic morphology of PbTiO 3 . All the primary particles in the TEM appeared to be substantially less than 0.02 micron. This observation is in accord with the high surface area, 131 m 2 /g, of the product. Further, this observation confirms the contention, made previously, that the hydrothermal treatment of the insoluble divalent cation oxide or hydroxide with a substantial molar excess of hydrous TiO 2 gives perovskites with very small primary particle sizes.
  • TEM's of the products of Examples XXV, XXVI and XXVII showed that the primary particles of the products had a variety of shapes ranging from spheres to cubes to rectangular platelets. Further, the primary particle size distributions were fairly broad and the primary particles were somewhat aggregated. The diversity of shapes is attributed to the high Pb content of the coforms.
  • the Ba-contents of the filtrates are higher than the desired value of about 4 g/L. Nevertheless, as shown by the X(II)/Y(IV) mole ratio, the observed deviation from stoichiometry is small in the case of Example XXVIII and, within the precision of the data, negligible in the case of Example XXIX. As in the synthesis of BaTiO 3 , product surface area increases with decreased Ba(OH) 2 addition time.
  • Example XXVIII had a primary particle size of about 0.13 micron while that of Example XXIX had a size of about 0.07 micron.
  • the primary particle size distributions were narrow and the products appeared to be dispersible. Moreoever, the TEM's, at the appropriate magnifications, resemble TEM's of barium titanate of FIG.1.
  • a STEM--energy dispersive X-ray analysis of the product of Example XXIX demonstrated that all the primary particles contained Pb, Ba and Ti at comparable levels.
  • Examples X to XXIX either Ba(II) in BaTiO 3 was partially replaced by a second divalent cation or Ti(IV) was partially replaced by a second tetravalent cation and relatively simple coforms were produced. More complex coforms were also synthesized by the simultaneous partial replacement of both the Ba(II) and Ti(IV) by one or more divalent and tetravalent cations. The systhesis procedure employed was identical with that used to make the simple Ca(II) and the Pb (II) contained coforms. In addition, it was found that the same set of parameters which controlled either BaTiO 3 or simple coform morphologies could be used to control the morphologies of the more complex coforms.
  • slurries containing the tetravalent hydrous oxides, preferably coprecipitated, and PbO or Pb(OH) 2 and/or Ca(OH) 2 were heated to 200° C. Thereafter, the slurries were cooled to a specified temperature, here called the synthesis temperature, and preheated Ba(OH) 2 solution at a temperature between 70° C. and 110° C., containing a Sr/Ba mole ratio of about 0.01, was introduced into the slurry in specified time interval. After holding at temperature for about 20 to 30 minutes the slurry temperature was raised to a final temperature of 200° to 250° C.
  • the exact final temperature selected will depend on coform composition, time, Ba(II) concentration in the aqueous phase and, probably, coform primary particle size.
  • coform composition time, Ba(II) concentration in the aqueous phase and, probably, coform primary particle size.
  • coform primary particle sizes in the range of 0.1 to 0.2 micron and Ba(II) concentrations of 3 to 10 g/L an increase in the final treatment temperature from 200° to 250° C. over a period of about 30 minutes increased the value of X(II)/Y(IV) by about 0.02 units. Accordingly, the selection of the final treatment temperature can be quickly established by means of a few experiments.
  • Table XIV is listed the molar quantities of the reactants employed for each coform, the initial slurry volume, V i , the total slurry volume after Ba(OH) 2 addition V f , the synthesis temperature employed, the Ba(OH) 2 addition time, the final treatment temperature used (not necessarily the optimum final temperature), the X(II)/Y(IV) mole ratio in the solid phase, product area and, from TEM data, an estimate of coform primary particle size, size distribution and dispersibility.
  • the SR(II) in the coform represents about 1 to 1.5 mole percent of the divalent cation content of the coform. For simplicity, this Sr is included in the Ba mole fraction in the coform.
  • Example XXXI As shown by the X(II)/Y(IV) mole rations in Table XIV, the coforms are either stoichiometric (within the precision of the analytical data) or very nearly stoichiometric. Further, apart from the product of Example XXXI, all products have primary particle sizes in the range of 0.05 to 0.2 micron, size distributions which are narrow or moderately narrow and appear to be dispersible.
  • the product of Example XXXI is polydisperse. Its polydispersity is attributed to the combination of high synthesis temperature and the relatively prolonged Ba(OH) 2 addition time of 2.9 minutes.
  • Example XXX When the Ba(OH) 2 addition time is reduced to 0.2 minutes at 200° C., as in Example XXX, a product with a moderately narrow size distribution is obtained. Because of the sensitivity of product primary particle size distribution to Ba(OH) 2 addition times at 200° C., a lower synthesis temperature is preferred.
  • Example XXXV The dispersibility of the product of Example XXXV (Table XIV) was also studied. With this sample it was found that less than 10 weight percent of the sample had a size greater than 0.25 micron and over 65 weight percent of the sample was less than 0.1 micron in size. These results demonstrate that coforms with TEM primary particle size of about 0.06 micron can also be dispersed.
  • a series of cobalt(II) and niobium(V) doped barium titanate products were prepared.
  • a stock 1M cobalt acetate solution was used as the cobalt source.
  • a hydrous niobium oxide wet cake which, on drying, had a nitrogen surface area of about 220 m 2 /g was used as the source of niobium(V).
  • Doping was accomplished by addition of the requisite amounts of the cobalt solution and/or the hydrous niobium oxide wet cake to 0.2 moles of hydrous titania.
  • the resulting slurry volumes were adjusted to 0.64 liters and then treated in an analogous manner to that used in the synthesis of the barium titanate of Example V.
  • Example XVII The amounts of the dopants employed in each example together with the analytical results obtained are summarized in Table XVII.
  • Example XLI When both Nb(V) and Co(II) are employed as dopants, as in Example XLI, the Ba(II)/Ti(IV) again exceeds unity. Finally, in the washed product of Example XLII, the Ba(II)/Ti(IV) ratio, within the precision of the data, is unity although the sample contains Nb(V). In this example, it is likely that washing reduced the Ba(II) content of the product.
  • the present examples show that the hydrothermal synthesis process of this invention can be used to dope barium titanate with a large variety of dopants without substantially affecting product morphology. Further, it is apparent that the procedures employed to dope barium titanate can be readily extended to doping coforms.

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MX169723B (es) 1993-07-21
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GB2193713A (en) 1988-02-17
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GB8715837D0 (en) 1987-08-12
FR2601352B1 (fr) 1995-06-30
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CN87105548A (zh) 1988-02-24
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IE871877L (en) 1988-01-14

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